Their preclinical research indicates that antitumoral immune stimulation by intratumoral gene therapy may be effective for the treatment of advanced colorectal cancer. Interleukin-2 (IL2) is a potent cytokine capable of inducing strong cellular immune responses against a variety of antigens. The Herpes Simplex Virus thymidine kinase (HSV-tk) is an enzyme which is not indigenous to mammalian cells and can phosphorylate the antibiotic ganciclovir which becomes incorporated into elongating DNA strands and produces cell death. Using an orthotopic murine model for pre-established metastatic colon cancer in the liver, we have shown that intratumoral injection of a combination of two adenoviral vectors (ADV-mIL2 and ADV-tk expressing the IL2 and HSV-tk genes respectively) together induced antitumor immunity, produced tumor regression and significantly prolonged survival. The induced antitumor immunity was systemic, mediated primarily by cytolytic T-lymphocytes (CTL) and rejected subsequent challenge of parental tumor cells implanted at distant sites. To test if similar antitumor immune responses can be induced in patients with metastatic colorectal cancer, we propose a Phase I trial where two vectors (ADV-hIL2 expressing the human IL2 gene and ADV-tk) will be mixed together and injected into metastatic tumors in the liver by percutaneous placement of a skinny needle under sonographic guidance. 4 hours later, ganciclovir will be administered intravenously twice daily for seven days. The dose of ADV-hIL2 will be escalated while the dose of ADV-tk will be fixed at 1011- pfu. The ADV-tk dose was based on a Phase I trial which we have completed of ADV-tk administered in the same manner in patients with metastatic colorectal cancer to the liver and followed by intravenous ganciclovir. No serious toxicities were encountered at doses from 109 to 1011- pfu. The proposed trial of the vector combination will evaluate toxicity, antitumor immunity, tumor response and disease progression. The immunologic endpoints are based on state-of-the-art methodologies and will help to define the relationship between antitumor CTL immune responses and tumor regression. Successful completion of the proposed trial will provide evidence to support further Phase II efficacy trials on active genetic immunization by intratumoral gene therapy as a new treatment modality for patients with metastatic colorectal cancer.